Reverse osmosis membranes have been prepared from a wide variety of known or preformed polymeric materials. In preparation of such membranes the polymer is typically dissolved in a suitable solvent, cast in the form of films or fibers, and quenched in water to form asymmetric membranes. These membranes include numerous polyamide and polyimide-type membranes that show utility in reverse osmosis desalination processes. In the book Reverse Osmosis and Synthetic Membranes, National Research Council of Canada, 1977, by S. Sourirajan, Chapter 9 by P. Blais presents an extensive list of polyamide membranes, including their fabrication and properties. These polyamide membranes are additionally described in U.S. Pat. Nos. 3,567,632, 3,600,350, 3,687,842, 3,696,031, 3,878,109, 3,904,519, 3,948,823, 3,951,789, and 3,993,625. These polyamide membranes are presently understood to be substantially linear polymers synthesized in a prior operation, then cast or extruded into permselective membranes by solvent processes. Polyamide membranes made in this manner appear to exhibit rather low fluxes to water in reverse osmosis desalination processes, as listed in Table 2 of the above-cited book, such that these polyamide membranes have found practical use only in the form of hollow fine fibers as according to the description in U.S. Pat. No. 3,567,632.
In addition, polyamide composite membranes, suitable for use in reverse osmosis desalination processes, have been prepared by condensation reactions in situ on a preformed support film. Examples of such membranes, and their preparation, are described in U.S. Pat. Nos. 3,744,642, 3,951,815, 4,005,012 and 4,039,440. For an example of an acid-catalyzed, in situ polymerization on the support, see U.S. Pat. No. 3,926,798. For an example of the crosslinking of a preformed semi-permeable polybenzimidazole membrane, see U.S. Pat. No. 4,020,142. Permselective membranes made by this thin film composite approach have, in some cases, exhibited greatly improved fluxes relative to preformed polyamides subsequently cast or extruded into membrane form by solvent processes. The aforementioned U.S. Pat. No. 3,744,642 contains a detailed description of desalination membranes prepared by interfacial condensation reactions.
However, the membranes of the prior art, whether prepared from preformed polymers or by in situ reactions, have oftentimes exhibited one or more other deficiencies such as low salt rejection, low durability or resistance to compression, sensitivity to extremes of pH or temperature, and lack of resistance to microbial attack or oxidation by chlorine in the feed water. Lack of resistance to chlorine in the feed water is a particularly noteworthy deficiency in permselective polyamide membranes. According to U.S. Pat. No. 3,951,815, the site of attack by chlorine on polyamide membranes is the amidic hydrogen atom present in the --CO--NH-- group. In compositions such as the polypiperazineamides described in U.S. Pat. Nos. 3,687,842, 3,696,031, and 3,951,815, resistance to chlorine in feed waters appears to have been adequately demonstrated; however, such resistance to attack by chlorine is believed to be atypical.
It would appear that permselective polyamide membranes could be obtained by condensation polymerization of diacyl halides with secondary diamines. Theoretically, the resulting polymeric products would be devoid of amidic hydrogen and would therefore be expected to be insensitive to chlorine in the feed water. Representative polyamides such as this have been prepared from piperazine and its derivatives as described in U.S. Pat. Nos. 3,687,842 and 3,696,031 and NTIS Rept. No. PB253 193/7GA.
These compositions exhibited permeate fluxes of 2.4 to 650 liters per square meter per day (1/m.sup.2 -day) at a pressure of 80 atmospheres toward saline feed solutions containing 5,000 to 10,000 ppm sodium chloride. These fluxes are generally uneconomical for use in reverse osmosis desalination processes (except in the form of hollow fine fibers).
Currently, process economics indicate a need for membrane fluxes of 600 to 800 1/m.sup.2 -day at pressures of 55 to 70 atmospheres for seawater feed (35,000 to 42,000 ppm total dissolved salts). For brackish waters containing 3,000 to 10,000 ppm salts, economically attractive membranes must provide permeate fluxes of 600 to 800 1/m.sup.2 -day at pressures of only 25 to 40 atmospheres. While specific reverse osmosis applications for permselective membranes may deviate from these requirements, such membranes will not achieve broad commercial applicability unless they meet these criteria. A need therefore remains for permselective polyamide membranes which combine the properties of chlorine resistance and high flux as well as the other properties mentioned above.